Molecular engineering of dihydroxyanthraquinone-based electrolytes for high-capacity aqueous organic redox flow batteries

Aqueous organic redox flow batteries (AORFBs) are a promising technology for large-scale electricity energy storage to realize efficient utilization of intermittent renewable energy. In particular, organic molecules are a class of metal-free compounds that consist of earth-abundant elements with good synthetic tunability, electrochemical reversibility and reaction rates. However, the short cycle lifetime and low capacity of AORFBs act as stumbling blocks for their practical deployment. To circumvent these issues, here, we report molecular engineered dihydroxyanthraquinone (DHAQ)-based alkaline electrolytes. Via computational studies and operando measurements, we initially demonstrate the presence of a hydrogen bond-mediated degradation mechanism of DHAQ molecules during electrochemical reactions. Afterwards, we apply a molecular engineering strategy based on redox-active polymers to develop capacity-boosting composite electrolytes. Indeed, by coupling a 1,5-DHAQ/poly(anthraquinonyl sulfide)/carbon black anolyte and a [Fe(CN)6]3−/4− alkaline catholyte, we report an AORFB capable of delivering a stable cell discharge capacity of about 573 mAh at 20 mA/cm2 after 1100 h of cycling and an average cell discharge voltage of about 0.89 V at the same current density.

Operando FTIR tests of 2,6-DHAQ and 1,5-DHAQ. Evolution of the FTIR spectra of a, 2,6-DHAQ and b, 1,5-DHAQ during reduction and oxidation processes in the first cycle of CV scan. ΔT is the transmittance difference of the sample relative to that before CV scan. Figure 10. 1 H NMR spectrum of 2,6-DHA in DMSO-d6. * Trimethylsilane (TMS) reference peak, ** Residual DMSO in DMSO-d6, *** Residual H2O in DMSO-d6. The synthesis of 2.6-DHA was carried out by following the reported procedure. 1 Supplementary Figure 11. Comparison of 2,6-DHAQ-based compounds. FTIR spectrum of the electrolyte at the end of resting, and the spectra of 2,6-DHAQ 2-, 2,6-DHAQ and 2,6-DHA. All tests were conducted at 25 ±1 °C. Figure 13. The hydrogen bond formation analysis. Surface electrostatic potential maps of a, four individual molecules (two 2,6-DHAQ 2and two H2O) and b, 2(2,6-DHAQ 2-)·2H2O complex. Supplementary Figure 17. Operando FTIR spectra of 1,5-DHAQ during the second and third CV scans.ΔT is the transmittance difference of the sample relative to that before CV scan. All the tests were conducted at 25 ±1 °C. Figure 18. a, Evolution of the Operando FTIR spectra of 1,5-DHAQ during reduction and resting processes. b, ΔT vs. time at 1634 cm -1 and 3250 cm -1 . ΔT is the transmittance difference of the sample relative to that before CV scan. The test was conducted at 25 ±1 °C. Figure 19. Comparison of reduction currents of 1,5-DHAQ during operando FTIR tests. Replot of the reduction current of 1,5-DHAQ during the three consecutive CV scans of FTIR testing. Replot means the currents were overlapped by three reduction process of operando FTIR tests in Supplementary Figures 7 Figure 21. Voltage profiles of the last cycle of 1,5-DHAQ and 2,6-DHAQ (86 th cycle for 2,6-DHAQ and 73 rd cycle for 1,5-DHAQ). The current density was 20 mA/cm 2 . All tests were conducted at 25 ±1 °C. battery testing at the oxidized state. Continued cycling leads to the appearance of additional peaks in the 1 H NMR spectrum in D2O, which belongs to the dimer caused by the reduced 2,6-DHAQ. The structure of 2,6-DHAQ dimer was reported by Goulet et al. 1 The battery was cycled for 86 cycles at the current density of 20 mA/cm 2 and 25 ±1 °C. Figure 23. Ex situ 1 H NMR spectra (500 MHz, D2O) of 1,5-DHAQ before and after battery testing at the oxidized state. No additional peaks are observed after battery testing, suggesting the highly stability of reduced 1,5-DHAQ. The battery was cycled for 73 cycles at the current density of 20 mA/cm 2 and 25 ±1 °C. Figure 24. CVs of 5 mM diluted 1,2-DHAQ anolyte before and after cycling experiments with a scan rate of 50 mV/s. The 1,2-DHAQ was at the oxidized state for CV tests and the battery was conducted for 77 cycles at the current density of 20 mA/cm 2 , after which the anolyte was diluted to conduct the CV test. All the tests were conducted at 25 ±1 °C. Figure 25. Ex situ 1 H NMR spectra (500 MHz, D2O) of 1,2-DHAQ before and after battery testing at the oxidized state. Continued cycling leads to the appearance of additional peaks in the 1 H NMR spectrum in D2O, which belongs to the dimer caused by the reduced 1,2-DHAQ. The structure of 1, 2-DHAQ dimer was tentatively confirmed by presuming it similar to 2,6-DHAQ dimer. The battery was cycled for 77 cycles at the current density of 20 mA/cm 2 and 25 ±1 °C. Figure 27. 1,5-DHAQ reduction kinetics in rotating disk electrode (RDE). a, Linear sweep voltammograms of 1 mM 1,5-DHAQ in 1 M KOH on a glassy carbon electrode at rotation rates between 300 and 2100 rpm. The small reduction wave might be caused by oxygen. The dissolved oxygen can oxidize the reduced DHAQ which regenerates DHAQ with an EC process, leading to an increase in current (small reduction wave) at around -0.7 V. b, Levich plot (limiting current versus square root of rotation rate in rad/s) of 1 mM 1,5-DHAQ in 1 M KOH. The slope yields a diffusion coefficient for the oxidized form of 1,5-DHAQ of 7.24 × 10 -6 cm 2 /s. c, Koutecky-Levich plot (reciprocal current versus inverse square root of rotation rate in rad/s) of 1 mM 1,5-DHAQ in 1 M KOH at different overpotentials. d, Fitted Tafel plot of 1 mM 1,5-DHAQ in 1 M KOH. The rate constant is calculated to be 1.29 × 10 -3 cm/s. All the tests were conducted at 25 ±1 °C. The PAQS polymer is insoluble in any solvent, thus solid-state 13 C NMR spectrum was performed to confirm its structure. Due to the strong rigidity of the polymer chains, the spectral line is significantly widened, and three spinning side bands appear around the main characteristic bands.    The testing time is calculated without considering the capacity fade and molecule utilization during the whole cycling process, so the actual time is less then calculated value, here we used "around" to indicate the results. Without the indicator "around", then the testing time is indicated in the reference paper. And the demonstrated volumetric capacity is based on the first cycle without considering the capacity fade. For 1,5-DHAQ-PAQS/CB system, the tests were conducted at 25 ±1 °C.

Supplementary Note 1. The adsorption energy calculation of different complex.
The energy of different compounds was summarized in Supplementary Table 2. The adsorption energy of one H2O on one 2,6-DHAQ 2is calculated by = , · − , − / , which are -0.310, -0.305, -0.300 and -0.297 eV for 2,6-DHAQ 2-·H2O, 2,6-DHAQ 2-·2H2O, 2,6-DHAQ 2-·3H2O and 2,6-DHAQ 2-·4H2O, respectively. The negative adsorption energy means that H2O can spontaneously adsorb onto 2,6-DHAQ 2-. Considering the adsorption energy of one water molecule onto one 2,6-DHAQ 2is the largest, here we use 2,6-DHAQ 2-·H2O for the further analysis.  Table 3). Considering the symmetric structure of reduced 1,5-DHAQ, the protonation process may take place at two positions as shown in Supplementary Figure 15. The energy of Structure I is lower than Structure II, suggesting Structure I is more stable and is thus used for the following analysis. The free energy change of the protonation process of Structure I was calculated based on the following equation in 1 M KOH solution: 1,5 _ + → 1,5 _ + The pH of the electrolyte would vary with the different protonation process of 1,5-DHAQ. If there isn't protonation, then the protons from 0.1 M 1,5-DHAQ will all react with OHin the electrolyte, resulting in a pH of 13.9 (without considering the volume change); if the 1,5-DHAQ is totally protonated, then there will be no proton dissolved into the solution and the pH of 1 M KOH remains 14. Thus, the pH could be in the range of 13.9-14 after dissolving 0.1 M 1,5-DHAQ in 1 M KOH. Then the ΔG was calculated with pH corrections: 74 where kB is the Boltzmann constant and T is the temperature. This leads to a ΔG>0.81 eV, indicating the protonation is not energetically favorable. Instead, the reduced 1,5-DHAQ would become deprotonated at such a high pH.
The above seemingly results in a pH-independent redox potential of 1,5-DHAQ, while the CV measurement in Figure S3 indicates a negative shift of the redox potential at high pH. This could be rationalized by the changes of activity coefficient. Based on the Nernst equation: Where is the standard potential, is the activity of the reduced and oxidized species, is the activity coefficient of the reduced and oxidized species, is the concentration of reduced and oxidized species, and is the formal potential:

Supplementary Note 3. The effect of driving force on the SMRT
For SMRT reactions, the driving force for reduction and oxidization of solid capacity boosting materials is governed by the potential difference between the molecule and solid capacity boosting materials. 2

Supplementary Note 4. Head loss analysis of adding granules into the tank.
One thing needs to mention is the head loss of adding granules into the tank. As those studied in packed bed reactors, the presence of solid granules in the tank would induce a pressure drop of the fluid, which leads to additional energy loss of pump. The pressure drop is related to a few factors of the media in the storage tank, such as the porosity and tortuosity of solid granules, packing (loading) of the solid materials, flow rate, etc., which involve extensive chemical engineering optimizations. We hope we could address this in a larger-scale device in future studies.

Supplementary Note 5. Experimental evaluation of theoretical volumetric capacity of PAQS/CB granules and volumetric capacity of 1,5-DHAQ/PAQS/CB systems.
The theoretical volumetric capacity of PAQS/CB granules were calculated based on the volume change after adding the equivalent capacity of PAQS/CB granules. As shown in Supplementary Figure Figure 40b), suggesting a chemical reduction process of PAQS through redox-targeting reaction between 1,5-DHAQ 2and PAQS forming PAQS 2-. For the oxidization process, the C-O signals of both 1,5-DHAQ 2and PAQS 2disappear, suggesting the oxidization process of 1,5-DHAQ 2on the electrode and chemical oxidization of PAQS 2through the redox-targeting reaction.